Why does the envelope of a star expand during the shell burning phase?
Answer
The hydrogen shell burning creates intense outward radiation pressure
When the core runs out of hydrogen, the layer of hydrogen surrounding the contracting helium core heats up and begins its own fusion process. This shell burning is significantly more energetic than the previous core fusion. The resulting intense radiation pressure pushes outward with immense force against the star's envelope. This pressure causes the outer layers to swell rapidly, moving significantly further away from the center and creating the bloated size characteristic of a red giant.
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